An Introduction to Computer Networks

1
An Introduction to Computer Networks
Lecture 3 Socket Programming

• University of Tehran
• Dept. of EE and Computer Engineering
• By
• Dr. Nasser Yazdani

2
Outline

• Socket programming
• What is a Socket?
• Why do we need sockets?
• Types of sockets
• Uses of sockets
• Example applications
• Code

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Socket programming

• Goal- Communication between two processes
• They use interface/services from the transport
  layer. The interface is called Application
  Programming Interface, API.

Server process
Client process
TCP/UDP
TCP/UDP
Socket API
IP
IP
Ethernet Adaptor
Ethernet Adaptor
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What are Sockets?

• It is an API between applications and network
• protocol software provided by the OS Functions it
  provides
• Define an end- point for communication
• Initiate and accept a connection
• Send and receive data
• Terminate a connection gracefully
• Supports multiple protocol families
• Examples Unix inter- process communication,
  TCP/ IP
• Only Internet sockets will be covered in this
  lecture
• Berkeley Sockets are the most common (from BSD
• Unix)

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Type of Sockets

• Two different types of sockets
• stream vs. datagram
• Stream socket ( a. k. a. connection- oriented
  socket)
• It provides reliable, connected networking
  service
• Error free no out- of- order packets (uses
  TCP)
• applications telnet/ ssh, http,
• Datagram socket ( a. k. a. connectionless
  socket)
• It provides unreliable, best- effort networking
  service
• Packets may be lost may arrive out of order
  (uses
• UDP)
• applications streaming audio/ video
  (realplayer),

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Socket usages

• Network applications use sockets at some level,
  often using higher level protocols on top of
  sockets
• File transfer apps (FTP), Web browsers (HTTP),
  Email (SMTP/ POP3), etc
• Simplify and expedite application development
  process
• Most exploits client-server model.

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A generic server (FTP)

• Wait for connections on a port
• When a client connection comes in,
• loop
• Read in the clients request
• Read data from a file
• Send the data to the client
• Disconnect when we have reached EOF

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A generic client, high level

• Connect to a given server
• loop
• Send a request to the server
• Read servers response
• Read servers data
• Disconnect when we have reached EOF

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Client- Server communication (TCP)
server
bind() to a receiving port
Accept() connection
socket()
listen () to socket
send() recv()
client
bind() to any port
send() recv()
socket()
connect () to server
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Socket

• Transport protocol needs the following
  information for multiplexing/demultiplexing.
• (source port, source address, destination port,
  destination address)
• Socket- the end point of connection. Process
  read/write from/to socket. A socket is specified
  by a socket descriptor.
• struct sockaddr_in
• u_char sin_family / Address Family /
• u_short sin_port / Port number /
• struct in_addr sin_addr / IP address /
• char sin zero8 / unused /

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TCP Server program

• Make a socket
• include ltsys/ types. hgt
• include ltsys/ socket. hgt
• int fd, newfd, nbytes, nbytes2
• char buf512, response512
• struct sockaddr_in srv
• fd socket(AF_INET, SOCK_STREAM, 0)

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TCP Server program

• The socket was created now bind it to a port and
  host.
• srv.sin_family AF_INET
• srv.sin_port htons(80)
• srv.sin_addr.s_addr inet_addr(128.2.15.9'')
• bind(fd, (struct sockaddr) srv, sizeof(srv))
• Now sit and listen
• listen(fd, 5)
• Now, accept any connection. First, clear the
  structure.
• struct sockaddr_ in cli
• int cli_ len
• memset( cli, 0, sizeof( cli)) / clear it /
• newfd accept(fd, (struct sockaddr) cli, cli
  len)

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TCP Server program

• Now it can read from socket, newfd, and write to
  socket, newfd.
• int BUF_ SIZE 1024, bytesrecv 0
• char buf BUF_ SIZE
• / receives up to BUF_ SIZE bytes from sock and
  stores them in buf. /
• bytesrecv recv( newfd, buf, BUF_ SIZE, 0)
• / send up BUF_ SIZE bytes /
• bytesrecv send( newfd, buf, BUF_ SIZE, 0)
• At the end, we need to close both sockets by
  close command.
• close( newfd) / closes the socket newfd /

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TCP client program

• int fd, newfd, nbytes, nbytes2
• char buf512, response512
• struct sockaddr_in srv
• fd socket(AF_INET, SOCK_STREAM, 0)
• The same as server. Now, it needs to connect to
  server.
• srv.sin_family AF_INET
• srv.sin_port htons(80)
• srv.sin_addr.s_addr inet_addr(128.2.15.9'')
• connect(fd, (struct sockaddr) srv,
  sizeof(srv))
• Connect is blocking and send a SYN signal and is
  blocked until receive SYNACK, (three way
  handshaking)
• It can start now reading and writing
• sprintf(request, Here's my request'')
• nbytes2 write(fd, request, strlen(response))
• close(fd)

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Client- Server communication (UDP)
server
recvfrom () sendto()
bind() to a receiving port
socket() to create socket
client
socket() to create scoket
recvfrom () sendto ()
bind() to any port
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UDP Server/client program

• / fill in declarations /
• fd socket(AF_INET, SOCK_DGRAM, 0)
• The socket was created now bind it to a port and
  host exactly the same way as TCP.
• Now listen no connection, start reading from the
  port by
• nbytes recvfrom(fd, buf, sizeof(buf), 0,
  (struct sockaddr) cli, cli len)
• UDP client is the same TCP, except instead of
  read/write, it uses
• nbytes sendto(fd, request, sizeof(request), 0,
  (struct sockaddr) srv, sizeof(srv))